As it is well known, the transmission of motion could be implemented using rigid mechanisms, compliant mechanisms with a fixed compliance, or a combination of these.
The rigid transmission mechanisms have the disadvantage of reflecting the overall inertia of the motor part to the impacting part, amplified by the square of the gear ratio. As a consequence, motions that guarantee a level of risk lower than a maximum given threshold are subject to limitations on the maximum admissible velocity in the case of rigid transmission.
Deformable transmission mechanisms with a fixed compliance, with a noticeable and constant damping, are generally obtained through the interposition of an elastic or a viscoelastic element between the motor shaft and the joint shaft, with deliberately high compliance characteristics.
The mechanisms have a disadvantage in that the joint stiffness rigidity can be set only during the task preparation phase (off-line), and not during the execution phase of the same task (on-line).
Another drawback of their mechanisms is that in many applications they cannot be used for the lack of precision in positioning, due to the slowness that characterize the acceleration phases and because of the ease with which oscillations of the elements placed at the end of the transmission are triggered.
For example, there are devices that limit the torque or the force and, when suitably tuned, reduce the maximum opposable resistance in such a way to allow the moving parts to execute the task, but that are also capable of decoupling the moving parts upon the threshold of a given maximum resistance. However, the greatest biological damage in the case of an accident is always determined in the very first instants of impact. Therefore, by also setting the thresholds at the minimum for the task execution, a large amount of damage has already occurred once the limitation of force or rigidity occurs.
Therefore, the need is felt to supply a transmission mechanism with variable stiffness that is also rapidly controllable, and that guarantees safety in the event that the devices interact or could interact with people.